Data modulation

ABSTRACT

A modulator includes a symbol mapper that is configured to map respective bits sets of a bit sequence corresponding to a burst and including data, training, tail, and guard bits into respective symbols to form a symbol sequence of data, training, tail, and guard symbols. A vector precoder is configured to apply a vector precoding transformation to the data and training symbols to form precoded symbols. These precoded symbols are combined with the tail and guard symbols in a symbol processor to form a sequence of transmit symbols. The record precoding conducted by the modulator of a transmitter enables improved link performance without the cost of increased processing complexity of the receiver algorithm.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a 35 U.S.C. §371 national stage application of PCTInternational Application No. PCT/SE2009/051474, filed on 21 Dec. 2009,the disclosure and content of which is incorporated herein by referencein its entirety.

FIELD

The present invention generally relates to data processing, and inparticular to modulation of data to be transmitted in a radio-basedcommunication network.

BACKGROUND

The Global System for Mobile communication (GSM) is currently the mostpopular standard for mobile telephones in the world and has beencommercially deployed since the early 1990s. Although more recentstandards for mobile communication in radio-based communication networkshave been proposed, there is still an interest on the continuedimprovement of the GSM technology and its improvements, such as EnhancedData rates for GSM Evolution (EDGE) also denoted Enhanced General PacketRadio Service (EGPRS) in the art. This means that improvements to thehardware and spectral efficiencies are still actively being sought.

With the advent of EGPRS phase 2 (EGPRS2) the GSM technology is reachingsome of its limits in terms of complexity and performance. Firstly, theGSM physical layer uses single carrier modulation and highly timedispersive narrowband channels. Secondly, the need to increase the datarates and spectral efficiency has resulted in the introduction of higherorder modulations. However, equalization of digitally modulated signalsusing these higher order modulations is a very demanding task for thereceiver. The reason is that the computational complexity of thedemodulator increases exponentially with the size of the symbolconstellation of the modulation.

Given the time dispersion present in all GSM radio channels, the use ofsuboptimal receiver algorithms is unavoidable. Despite manysimplifications these algorithms are still highly complex.

U.S. Patent Application No. 2008/0225985 discloses a technique forenhancing the capacity of a wireless communication channel by modulatingdata with a modulation scheme and transforming the modulated data fromthe frequency domain to the time domain. Bits are then encoded in atimeslot independence upon the time domain version of the modulateddata. The document proposes using 40 pre-specified time domain valuesout of the 156 available in a burst, which requires 40 of the frequencydomain symbols to be left as variables and cannot therefore carry anydata. This process is, however, complicated and has high computationalcomplexity.

SUMMARY

There is therefore a need for a solution that allows furtherimprovements of the GSM technology but does not introduce highcomplexity in the receiver or transmitter of the communication devices.

It is an objective to provide an improved modulation in connection witha transmission chain.

It is a particular objective to provide multi-channel modulation toGSM-based communication networks.

These and other objectives are met by embodiments as disclosed herein.

Briefly, a modulator comprises a symbol mapper, to which a bit sequencecorresponding to a radio burst and comprising data, training, tail andguard bits is input. The symbol mapper maps respective sets of at leastone bit of the bit sequence into respective symbols to form a symbolsequence of data, training, tail and guard symbols. A vector precoder ofthe modulator processes the data and training symbols by applying avector precoding transformation to these symbols to form correspondingprecoded symbols. The precoded symbols are combined with the tail andguard symbols from the symbol mapper in a symbol processor to form asequence or burst of transmit symbols, which can then be upsampled,filtered, upmixed and amplified and transmitted into the air as a radiosignal during a time slot.

The vector precoding of the data and training symbols enables usage ofhigher order modulations as compared to the traditional modulation ofGSM-compatible devices but without the associated drawbacks of increasedreceiver algorithm complexity as in the prior art.

An aspect of the embodiments also relates to a data modulation methodinvolving mapping respective sets of at least one bit of an input bitsequence corresponding to a radio burst and comprising the data,training, tail and guard bits. The result of the mapping is a sequenceof data, training, tail and guard symbols. A vector precodingtransformation is applied to the data and training symbols to formcorresponding precoded symbols that are combined with the tail and guardsymbols to form a sequence of transmit symbols.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention, together with further objects and advantages thereof, maybest be understood by making reference to the following descriptiontaken together with the accompanying drawings, in which:

FIG. 1 is a schematic overview of a radio-based communication network inwhich embodiments can be implemented;

FIG. 2 schematically illustrates a transmitting chain according to anembodiment;

FIG. 3 schematically illustrates a modulator according to an embodiment;

FIG. 4 is a flow diagram illustrating a modulation method according toan embodiment;

FIG. 5 is a flow diagram illustrating an additional method step of themodulation method in FIG. 4 according to an embodiment;

FIG. 6 is a flow diagram illustrating an additional method step of themodulation method in FIG. 4 according to another embodiment;

FIG. 7 is a flow diagram illustrating a transmitting method according toan embodiment;

FIG. 8 is a flow diagram illustrating an embodiment of the symbolmapping step of the transmitting method in FIG. 7;

FIG. 9 is a flow diagram illustrating an embodiment of the pulse shapingstep of the transmitting method in FIG. 7; and

FIG. 10 is a diagram comparing downlink throughput between an EGPRS2-Asystem and an embodiment.

DETAILED DESCRIPTION

Throughout the drawings, the same reference numbers are used for similaror corresponding elements.

The present embodiments relate to data processing in connection with atransmitter useful in a wireless, radio-based communication network. Inparticular, embodiments as disclosed herein relate to modulation of datain the transmitter.

The embodiments introduce a new data modulation that is particularlyapplicable to radio-based communication networks employing the GSMtechnology and its developments, i.e. EDGE/EGPRS including the recentlyproposed EGPRS2 technology. As is well known, GSM is a time divisionmultiple access (TDMA)/frequency division multiple access (FDMA) system,where the allotted frequency band is divided into 200 kHz wide channelsand time is split into time slots having a length of 15/26 ms, i.e.about 576.9 μs.

FIG. 1 is a schematic overview of a radio-based communication network 1comprising a base transceiver station or base station 20 serving acertain geographical area, typically denoted cell 10 in the art. Mobiletelephones or devices 30-70 and other communication devices presentwithin the cell 10 can be involved in communication services using thebase station 20 and the communication resources, i.e. time slots ofdefined frequencies, offered at the cell 10.

Embodiments introduce an extension to the basic TDMA/FDMA multiplexingof the GSM technology that achieves improved radio link performance butdoes not require the complex receiver algorithms of the proposed EGPRS2technology. Thus, increased system throughput and improved spectralefficiency can be obtained through the introduction of higher ordersymbol constellations but without the need for equalizers and thedemanding signal processing that follows from the introduction of suchhigher order symbol constellations according to the prior art.

The TDMA/FDMA multiplexing is extended by the introduction of discretetime channel partitioning (DTCP). DTCP is a type of multi-channelmodulation, which is well suited for digital implementation. DTCP refersto a choice of transmit basis vectors and defines the relation between adiscrete set of input samples and a discrete set of output samples bymeans of a linear transformation.

A preferred implementation of DTCP is vector precoding. Vector precodingcreates a set of independent channels by a judicious choice of thetransmit basis vectors. Vector precoding is further employed herein insuch a way that the time and frequency divisions used in GSM arepreserved. Additionally, frequency hopping which is commonly used in GSMto alleviate multipath fading can still be performed exactly as beforeby the introduction of vector precoding.

FIG. 2 is an illustration of a transmit chain according to an embodimentimplemented as a transmitter 200 or the transmit chain of a transceiverin a communication device. The transmitter 200 comprises four mainunits: a burst formatting unit 180, a modulator 100 according to theembodiments, an upmixer and amplifier 190 and a connected transmitantenna 195 or transmit antenna system. The modulator 100 is exemplifiedas a linear modulator 100 complemented with a vector precoder 120 andadditionally comprises a symbol mapper 110 and a symbol processor 130.

The symbol mapper 110 of the modulator 100 is configured to receive abit sequence corresponding to a radio burst, preferably from the burstformatting unit 180 as illustrated in FIG. 2. The bit sequence comprisesdata bits and training bits. The data bits represent the user code bitscarrying the payload or control data or information that is to betransmitted in the form a radio burst. Training bits are well known inthe art and are used for synchronization and channel estimation. The bitsequence preferably also comprises tail bits and guard bits. Tail bitsare traditionally employed for resetting the equalizer state in thereceiver and the guard bits allow power ramp up and down and for somepropagation time delay in the arrival of the radio bursts to ensure thatthe time slots do not collide with each other. Though, embodiments canuse a bit sequence without these tail and guard bits they are preferablyincluded in the bit sequence to ensure that the power versus timeprofile complies with the GSM specification to ensure that the signallevel is known at some particular time for measurement purposes in thetransmitter.

The bit sequence enters the symbol mapper 110, which processes the bitsequence by mapping respective sets of at least one bit of the bitsequence into respective symbols to form a symbol sequence. Thus, thesymbol mapper 110 receives a bit sequence comprising data bits, trainingbits and preferably tail bits and guard bits and output a symbolsequence comprising data or payload symbols, training symbols andpreferably tail symbols and guard symbols.

In an alternative embodiment, the symbol mapping operation of the symbolmapper 110 is conducted before formatting the symbol sequence into aburst. In this case the data bits, training bits and preferably the tailbits and guard bits are first mapped to respective data symbols,training symbols and tail symbols and guard symbols as described above.The resulting symbols are then formatted into a sequence correspondingto a radio burst, such as flank the data and training symbols with tailsymbols and guard symbols as described herein.

The data symbols and the training symbols from the symbol mapper 110 areinput to the vector precoder 120. The vector precoder 120 processesthese symbols by applying a vector precoding transformation to the datasymbols and the training symbols to form corresponding precoded symbolsor transmit versions of the data symbols and the training symbols.

The vector precoder 120 thereby performs a coordinate transformationthat induces channel partitioning.

The output from the vector precoder 120, i.e. the precoded symbols, iscombined with the tail and guard symbols from the symbol mapper 110 inthe symbol processor 130 to from a sequence or burst of transmitsymbols.

Embodiments of the present invention will now be described in moredetail in connection with implementation examples.

The burst formatting unit 180 receives the data bits and combines thesewith the training bits, tail bits and guard bits to form the bitsequence corresponding to a radio burst. This bit sequence has thefollowing structure after burst formatting:

$\left( \left. \quad{\underset{guard}{b_{1},\ldots\mspace{14mu},b_{\alpha}},\underset{tail}{b_{\alpha + 1},\ldots\mspace{14mu},b_{\beta}},\underset{data}{b_{\beta + 1},\ldots\mspace{14mu},b_{\chi}},\underset{training}{b_{\chi + 1},\ldots\mspace{14mu},b_{\delta}},\underset{data}{b_{\delta + 1},\ldots\mspace{14mu},b_{ɛ}},\underset{tail}{b_{ɛ + 1},\ldots\mspace{14mu},b_{\phi}},\underset{guard}{b_{\phi + 1},\ldots\mspace{14mu},b_{\phi}}} \right) \right.$

Alternatively, all guard bits can be provided at the end or at thebeginning of the bit sequence. This is the traditional organization ofbits in the normal burst of GSM. In the prior art, 26 training bits, 2×3tail bits and 8.25 guard bits are used to complement the 2×58 data bits.If the symbol mapper 110 employs a lower order of modulation, such asbinary phase-shift keying (BPSK), for respective sets of one bit, theabove mentioned bit lengths of respective portions of the bit sequencecan advantageously be used. However, embodiments as disclosed herein canadvantageously be used in connection with higher order modulations, inwhich sets of multiple, i.e. at least two, bits are mapped intorespective symbols, such as by means of quadrature PSK (QPSK), 8-PSK,4/16/32/64 or even 128 quadrature amplitude modulation (QAM) that mapsets of two (QPSK, 4 QAM), three (8-PSK), four (16 QAM), five (32 QAM),six bits (64 QAM) or seven bits (128 QAM) into a symbol.

The bit sequence is input to the symbol mapper 110 to map sets of one ormore bits into respective symbols drawn from a symbol constellation,such as one of the above-mentioned PSK and QAM symbol constellations.The symbol mapping, thus, results in the symbol sequence:(b ₁ , . . . ,b _(α) ,b _(φ+1) , . . . ,b _(φ))→g=(g ₁ , . . . ,g _(η))guard(b _(α+1) , . . . ,b _(β) ,b _(ε+1) , . . . ,b _(φ))→t=(t ₁ , . . . ,t_(ν)) tail(b _(β+1) , . . . ,b _(χ) ,b _(δ+1) , . . . ,b _(ε))→x=(x ₁ , . . . ,x_(D)) data(b _(χ+1) , . . . ,b _(δ))→s=(s ₁ , . . . ,s _(N) _(tr) ) trainingwhere g, t, x, s denote the (PSK/QAM) symbols that carry the guard,tail, data and training bits, respectively. The symbol sequencecomprises D data symbols and N_(tr) training symbols, with a total ofdata symbols and training symbols of N=D+N_(tr). The total number ofsymbols in the symbol sequence corresponding to a radio burst isK=N+η+ν. For a GSM/EDGE implementation, typical values of theseparameters are N=142, η=8 and ν=6.

The output of the symbol mapper 110 is, thus, the symbol sequence:[c ₁ , . . . ,c _(K) ]=[g,t,x,s]

With reference to the modulator embodiment illustrated in FIG. 3, themodulator 100 advantageously comprises a symbol interleaver 140implemented to intercalate the training symbols and the data symbols forsynchronization and channel estimation purposes. In the prior art, suchinterleaving is conducted by placing all the training symbols in themiddle of the data symbols to thereby form a first set of 58 datasymbols followed by 26 training symbols and then the remaining 58 datasymbols.

In a preferred implementation, the symbol interleaver 140 is configuredto interleave the training symbols among the data symbols to form atleast Q≧3 sets of data symbols separated by respective sets of at leastone training symbol. This means that the training symbols are preferablydistributed or spread out among the data symbols and not allconcentrated in the middle of the symbol sequence and thereby in themiddle of the radio burst. The particular location of the trainingsymbols has an impact on the receiver performance and thereby it isgenerally advantageous to distribute the training symbols and notconcentrate all these symbols in a single set. The particularinterleaving scheme employed by the symbol interleaver 140 can depend onvarious parameters, such as the code rate or presence of in-bandsignaling. This means that the employed interleaving scheme can, forinstance, be selected to reduce the overall channel estimation error. Itcan also be advantageous to interleave the training symbols within thedata symbols at positions that are expected to have highersignal-to-noise ratios (SNRs) as compared to other symbol positionswithin the sequence and the radio burst. An example implementation is touniformly distribute the training symbols among the data symbols.

The symbol interleaver 140 thereby outputs a vector z of length Nsymbols that is constructed from the data symbols x and the trainingsymbols s:

$z = {\left\lbrack {z_{1},\ldots\mspace{14mu},z_{N}} \right\rbrack^{T} = \underset{\overset{\updownarrow}{k{(1)}}\mspace{14mu}\ldots\mspace{14mu}\overset{\updownarrow}{n{(1)}}\mspace{14mu}\ldots\mspace{14mu}\overset{\updownarrow}{k{(p)}}\overset{\updownarrow}{n{(m)}}\mspace{14mu}\ldots\mspace{14mu}\overset{\updownarrow}{n{(N_{tr})}}\mspace{14mu}\ldots\mspace{14mu}\overset{\updownarrow}{k{(D)}}}{\left\lbrack {x_{1},\ldots\mspace{14mu},s_{1},\ldots\mspace{14mu},x_{p},s_{m},\ldots\mspace{14mu},x_{N_{tr}},\ldots\mspace{14mu},x_{D}} \right\rbrack^{T}}}$

The location of the training symbols is given by the indices(n(m))_(m=1) ^(N) ^(tr) and the location of the data symbols is likewisegiven by (k(p))_(p=1) ^(D). In other words z_(n(m))=s_(m) andz_(k(p))=x_(p).

The output vector from the optional but preferred symbol interleaver 140is forwarded to the vector precoder 120, where vector precoding isapplied to the vector z to form a new sequence of complex numbers Zrepresenting the precoded symbols.

In a preferred implementation, the vector precoding transformationemployed by the vector precoder 120 is a discrete transformation andmore preferably selected from the group consisting of discrete Fouriertransform (DFT), inverse DFT (IDFT), discrete cosine transform (DCT),inverse DCT (IDCT), discrete wavelet transform (DWT) and inverse DWT(IDWT).

The vector precoder 120 then applies a precoding matrix W to the vectorwith the interleaved training and data symbols to perform a coordinatetransformation that induces channel partitioning.

For example, in the case of IDFT precoding, W is the Fourier transformmatrix of size N×N. The entries in the matrix can be defined as or atleast be derivable from

${W_{m,i} = {\frac{1}{\sqrt{N}}{\mathbb{e}}^{{- j}\; 2\pi\;\frac{{({m - 1})}{({i - 1})}}{N}}}},$m=1, . . . , N, i=1, . . . , N, where m is a row counter and i is acolumn counter of the matrix. By employing IDFT precoding, the vectorprecoder 120 performs a coordinate transformation that yields astransmit basis vectors the eigenvectors of the radio channel matrix.These eigenvectors are nearly independent of the propagationenvironment.

The vector precoding operation conducted by the vector precoder 120 istherefore:Z=W ^(H)z

Multiplication by the matrix W^(H) can be implemented efficiently usingthe fast forward transform (FFT). W^(H) is an Hermitian matrix, i.e. asquare matrix with complex entries which is equal to its own conjugatetranspose. In other words the matrix element in the m-th row and i-thcolumn is equal to the complex conjugate of the element in the i-th rowand the m-th column. If the vector precoder 120 performs a coordinatetransformation using IDFT, the eigenvectors of the radio channel matrixwill be the transmit basis vectors. These eigenvectors are nearlyindependent of the propagation environment.

As another example, the vector precoder 120 could instead apply a vectorprecoding transformation that is based on DCT. In such a case, theprecoding matrix w is given by:

${W_{\;{m,i}} = {\sqrt{\frac{2}{N}\;}{\cos\left( {\frac{\pi}{2N}\left( {i - 1} \right)\left( {{2m} - 1} \right)} \right)}}},{m = 1},\ldots\mspace{14mu},N,{i = 1},\ldots\mspace{14mu},N$and the IDCT matrix is given by:

${W_{m,i} = {\sqrt{\frac{2}{N}}{a\left( {i - 1} \right)}{\cos\left( {\frac{\pi}{2N}\left( {i - 1} \right)\left( {{2m} - 1} \right)} \right)}}},{m = 1},\ldots\mspace{14mu},N,{i = 1},\ldots\mspace{14mu},N$where ${a(k)} = \begin{Bmatrix}\frac{1}{2} & {k = 0} \\1 & {1 \leq k \leq {N - 1}}\end{Bmatrix}$

In an optional but preferred embodiment, the modulator 100 comprises acyclic prefix processor 150 that operates on and processes the outputfrom the vector precoder 120. The cyclic prefix processor 150 adds acyclic prefix to the precoded symbols. The purpose of such cyclic prefixis to allow multipath to settle before the main data arrives at thereceiver.

A cyclic prefix processor 150 can be used in order to mitigate timedispersion and make the signaling less sensitive to time dispersion onthe radio channel. The cyclic prefix processor 150 is configured to adda cyclic prefix of length L≧0 symbols to the precoded symbols from thevector precoder 120. If L=0 no cyclic prefix is employed whereas anon-zero integer value of the parameter L indicates that the cyclicprefix processor 150 has a cyclic prefix to the precoded symbols. In apreferred implementation the cyclic prefix processor 150 is configuredto append the last L precoded symbols in the vector Z from the vectorprecoder 120 at the beginning of the vector to form a new vector Z^(p):Z ^(P) =└Z ₁ ^(p) . . . ,Z _(N+L) ^(p) ┘=[Z _(N−L) ,Z _(N−L+1) , . . .,Z _(N) ,Z ₁ ,Z ₂ , . . . ,Z _(N)]

The output from the cyclic prefix processor 150 is forwarded to thesymbol processor 130, which also receives the tail symbols and the guardsymbols from the symbol mapper 110.

In an alternative embodiment, the cyclic prefix processor 150 operatesas a cyclic postfix processor to instead add a cyclic postfix to theprecoded symbols. In a preferred implementation, the processor is thenconfigured to append the first L precoded symbols in the vector Z fromthe vector precoder 120 at the end of the vector to form a new vector.

In yet another approach that is in particular suitable with DCT asvector precoding transformation but can also be employed in connectionwith other vector precoding embodiments is to have a processor 150 thatadds both a cyclic prefix and a cyclic postfix to the precoded symbols.The prefix and the postfix can then be of the same length in terms ofthe number of symbols but can also be of different lengths.

It is anticipated by the embodiments that the processor 150 does notnecessarily have to add a cyclic prefix and/or postfix but can beconfigured to add a prefix and/or postfix, which may be but does nothave to be cyclic.

The symbol processor 130 forms a sequence of transmit symbols based onthe input data, where this sequence of transmit symbol is a sequence ofcomplex numbers:d=[d ₁ ,d ₂ , . . . ,d _(K+) ]=└g ₁ , . . . ,t ₁ , . . . ,Z ₁ ^(p) ,Z ₂^(p) , . . . ,Z _(N+L) ^(p) , . . . ,t ₈₄ , . . . ,g _(η)┘

Thus, the transmit symbols comprises the precoded symbols preceded withthe optional cyclic prefix flanked by tail symbols, which in turn may beflanked guard symbols. In an embodiment, ν is an even number and halfthe tail symbols precede the precoded symbols and the cyclic prefix andthe remaining half follow after the precoded symbols. In the case of anodd number, the odd tail symbol can either precede or follow theprecoded symbols. The guard symbols can also be distributed in thismanner with half of them before the first set of tail symbols and theremaining half following the second set of tail symbols. Any odd guardsymbol can then be placed in the first or second set of guard symbols.In an alternative embodiment, the guard symbols are not distributed intotwo sets. Instead all guard symbols are provided in the beginning of thesequence or at the end of the sequence.

The output from the symbol processor 130 is preferably input to anoptional pulse shaper 170 that modulate the sequence of transmit symbolsonto a carrier signal followed by upsampling, filtering, upmixing andamplification in an upmixer and amplifier 190 before transmission by theantenna 195 to the air using a linear modulator.

The modulator embodiments as disclosed herein are suitable forimplementation in a transmitter or in the transmitting chain of acommunication device to enable wireless transmission of data in aradio-based communication network. Through the operation of the vectorprecoder 120 of the modulator 100, a simplified receiver algorithm canbe utilized that does not have to be based on computationally complexTrellis-based equalizers. In clear contrast and due to the vectorprecoding conducted in the transmitter, inter symbol interference can beeliminated at the receiver. As a consequence, a simple signal model isobtained that can used to determine the log-likelihood ratios for thecoded user bits, i.e. the so-called soft values.

The embodiments can achieve radio link performances that are at least atthe levels obtainable by the EGPRS2 technology, though at substantiallyless complex receiver algorithms. This therefore results in increasedthroughput and improved spectral efficiency without any increasedcomplexity and cost of the receiver algorithm. As a side effect, lowerpower consumption per bit is expected both at the transmitter and at thereceiver. In particular, the lower complexity at the receiver reducesthe power consumption needs, which is highly advantageous forbattery-powered communication devices such as mobile devices.

The embodiments further provide a more robust physical layer as comparedto the proposed EGPRS2 technology by making the receiver performanceless sensitive to imperfections in both the transmitter and receiverchains, which is today a significant problem for the receivers. Inparticular for the higher order modulations introduced with EGPRS2, thesensitivity to any imperfections, distortions or noise introduced in theprocessing at the transmitter and/or receiver increases significantly.These problems are solved or at least mitigated by the embodiments.

The modulator 100 of the embodiments is advantageously implemented inbase stations of the communication network, such as the base station 20illustrated in FIG. 1. The modulator 100 can be implemented in legacyGSM/EDGE base stations since the solutions presented by the inventionare fully backward compatible with GSM/EDGE. As has been mentioned inthe foregoing, the TDMA/FDMA structure can be preserved, frequencyhopping can be performed as in the current GSM/EDGE-based communicationnetworks and the basic 13 MHz clock used as time base counter in bothbase transceiver stations and mobile devices still provides the timebase. Additionally, even already defined EGPRS/EGPRS2 logical channelsmay be re-used and modified by adding DTCP and vector precoding in themodulator, resulting in improved link performance, while re-usingexisting digital signal processing circuitry, algorithms andimplementations. Also specific radio frequency (RF) requirements such asout of band emissions and spectrum masks may be preserved.

The modulators employed in the user devices communicating with the basestation, i.e. the mobile devices, can also be designed according to theembodiments. Alternatively, vector precoding could be used only in thedownlink to thereby relax the need for modifying the transmitteralgorithms of all mobile devices. Thus, in such a case the downlink anduplink will be different with vector precoding employed by the basestations in the downlink transmissions but not necessarily by the mobiledevices for uplink transmissions. An asymmetric TDMA/FDMA/DTCP system isthus achieved.

Vector precoding is suitable for implementation in connection withhigher order modulation. In such a case, the symbol mapper 110 isconfigured to map sets of multiple bits into respective symbols. Thismeans that the throughput will increase since the amount of data interms of data or payload bits that can be transmitted during a radioburst is significantly increased as compared to using the traditionalnon-linear Gaussian minimum shift keying (GMSK) modulator used for voiceservices and some packet data services in existing GSM/EDGEcommunication networks.

The vector precoding is therefore suitable for usage in connection withcommunication services requiring high data rate transmissions, such assome data packet services. In such a case, the base station and thetransmitter can select whether to apply to the modulation of theembodiments or traditional modulation based on the particularcommunication service employed. Thus, for voice services and other delaysensitive communication services, the transmitter can performtraditional modulation using non-linear GSMK. In clear contrast,communication services having demands for higher data rates andgenerally using higher order modulation, such as QPSK, 8-PSK, 16 QAM, 32QAM, 64 QAM or 128 QAM, could use the vector precoding in connectionwith the linear modulator. Therefore selection of modulation algorithm,i.e. vector precoding or not and usage of low or higher ordermodulation, can be made at the transmitter based on the currentcommunication services.

The modulator 100 may therefore be equipped with a controller 160 thatcontrols the operation of the symbol mapper 110 and the vector precoder120. The controller 160 then determines the communication service forthe current data to be transmitted and then selects between employingnon-linear GMSK modulation or higher order modulation with or withoutvector precoding based on the determined communication service.

The selection whether or not to apply vector precoding as determined bythe controller 160 can also be based on the capabilities of the mobiledevices. The register over the mobile devices present in thecommunication network can then list whether the mobile devices supportlinear transformation modulation or not. In such a case, the basestation requests such information from the register or automaticallyreceives the information once it becomes the serving base station forthe mobile device. The controller 160 uses the information to determinehow modulation of the user code data should be conducted, such aswhether vector precoding should be employed or not based on thecapability information.

A further decision basis to employ by the controller 160 could be thesignal or link quality of the communication link between the basestation and a mobile device. Any of the known quality measures, such asSNR, determined by the base station or the mobile device and reported tothe base station can be used by the controller 160. For instance, if thecurrent signal quality is low as determined based on the qualitymeasure, i.e. below a predefined minimum quality threshold, thecontroller 160 could be configured to control the modulator 100 toperform modulation of user code data according to the traditional GSMKmodulation approach. However, if the current signal quality isacceptable, i.e. above the quality threshold, the controller 160activates the vector precoder 120 to thereby perform vector precoding ondata and training symbols from the symbol mapper 110 or the preferredsymbol interleaver 140.

The controller 160 can also be used in order to control the operation ofthe symbol mapper 110, the symbol interleaver 140, the vector precoder120 and the cyclic prefix processor 150. For instance, the controller160 can control the cyclic prefix processor 150 to use a selectedparameter L, i.e. the length of the (cyclic) prefix and/or postfix,based on control signaling from the controller 160. The parameter Lcould then be determined by the controller 160 based on the currentpropagation environment as determined from signal quality measurements.For instance, different cyclic prefix lengths are preferably employeddepending on whether the communication takes place indoors or outside inrelative free and unblocked terrain. A non-limiting example of L=5 canbe used to correspond to the typical GSM channel length.

The interleaving of the training symbols among the data symbols can beselected by the controller 160 based on different criteria as haspreviously been mentioned, e.g. code rate and presence of in-bandsignaling. This means that the controller 160 thereby may control thesymbol interleaver 140 to position the training symbols at positionsthat are most suited for the current signaling conditions.

Additionally, the parameters N, N_(tr) and ν, i.e. total number of dataand training symbols, number of training symbols and number of tailsymbols, could either be fixed or adjusted by the controller 160. Forinstance, the choice of the parameters N and N_(tr) depend on whetherbackward compatibility with EGPRS/EGPRS2 codes is required since theyaffect the total number of user coded bits that are accommodated in aradio burst. The parameter ν can similarly be chosen by the controller160 depending on the difficulty for the transmitter 200 to satisfy timemask requirements.

Depending on the particular choice of the parameters L, N, N_(tr) and νeach radio burst can accommodate a different number of user coded bitsthan that currently offered by the existing EGPRS/EGPRS2 coding schemes.In such a case, the burst formatting unit 180 or preferably a ratematching unit of the transmitter 200 can be used. The rate matching unitis not illustrated in FIG. 2 but is preferably implemented prior theburst formatting unit 180 to conduct the rate matching before burstformatting. The rate matching unit is then configured to conduct ratematching through additional puncturing, imputation of punctured codedbits or repetition of coded bits generated by the existing EGPR/EGPRS2codec.

As has been previously discussed, the embodiments are preferably used inconnection with higher order modulations. In such a case, one and thesame symbol constellation may be used for all bits that are mapped bythe symbol mapper 110 into respective symbols. However, it is alsopossible to control the symbol mapper 110 by the controller 110 to tunethe symbol mapping to increase the channel capacity. In such a case, thesymbol mapper 110 can be controlled to choose symbols from differentsymbol constellations during a single radio burst, for instance onesymbol may be QPSK whereas another symbol is 32 QAM. The symbol mapper110 consequently is configured and controlled to select, for eachrespective set of at least one bit in the bit sequence, a symbolconstellation to use for that respective set. The symbol constellationis preferably selected among a QAM symbol constellation and a PSK symbolconstellation. In a preferred implementation, the particular choice ofsymbol constellation can be based on the position of the respective setwithin the bit sequence and correspondingly the position of theresulting symbol within the symbol sequence. Since the radio channel hasa band-pass characteristic, some symbols in any given burst willgenerally have a higher average signal to noise ratio than others.Therefore, in order to optimize the channel capacity, it may beadvantageous to allow the simultaneous use of several symbolconstellations within one burst. Generally with mixed modulation schemethe weaker symbols should belong to the lower order symbol constellationas compared to other symbols in the symbol sequence. Two or moredifferent symbol constellation can be used in such a mixed modulationscheme.

The traditional GSM/EDGE channel has a transmit pulse bandwidth of 200kHz. This same pulse bandwidth can be used by the embodiments. However,the data rate can be increased by a factor M by increasing the transmitpulse bandwidth by the same factor. In such a case, the pulse shaper 170of the modulator 100 or alternatively implemented outside of themodulator 100 in the transmitter 200 is configured to modulate thesequence of transmit symbols from the symbol processor 130 onto acarrier signal having a bandwidth of M×200 kHz. M is consequently apositive integer equal to one or larger. If M>1 M 200 KHz adjacent radiochannels will be used for transmitting the radio burst. Embodimentstherefore opens up the possibility to use wider radio channels, thusincreasing the data rates, while keeping low the requirements on thecomplexity of the analog and digital signal processing platforms in thereceiver.

The embodiments as disclosed herein can re-use the already definedcodecs of EGPRS/EGPRS2 and thus the technique proposed herein has theadditional advantage of being transparent to higher layers.

The units 110 to 170 of the modulator and 110, 180, 190 of thetransmitter 200 may be implemented or provided as hardware or acombination of hardware and software. In the case of a software-basedimplementation, a computer program product implementing the modulator100 or the transmitter 200 or a part thereof comprises software or acomputer program run on a general purpose or specially adapted computer,processor or microprocessor. The software includes computer program codeelements or software code portions illustrated in FIGS. 2 and 3. Theprogram may be stored in whole or part, on or in one or more suitablecomputer readable media or data storage means such as magnetic disks,CD-ROMs, DVD disks, USB memories, hard discs, magneto-optical memory, inRAM or volatile memory, in ROM or flash memory, as firmware, or on adata server.

FIG. 4 is a flow diagram illustrating a data modulation method accordingto an embodiment. The method starts in step S1 where a bit sequencecorresponding to a radio burst is input. The bit sequence comprises databits corresponding to user code bits, also denoted payload bits.Additionally, training bits and preferably tail and guard bits areincluded in the bit sequence. Respective sets of at least one bit of thebit sequence is mapped into respective symbols in step S2 to form asymbol sequence of data symbols, training symbols and preferably tailsand guard symbols. A vector precoding transformation is applied in stepS3 to the data and training symbols to form corresponding precodedsymbols thereof. A next step S4 forms a sequence of transmit symbolsbased on the precoded symbols and the preferred tail and guard symbols.

A preferred embodiment of step S2 involves mapping respective sets ofmultiple bits of the bit sequence into respective symbols to form thesymbol sequence. In this embodiment there is a many-to-one relationshipbetween bits and symbols.

In an alternative approach, the data, training, tail and guard bits arefirst mapped into respective symbols and are then formatted andorganized into a burst before continuing further into the vectorprecoding step S3.

FIG. 5 is a flow diagram illustrating an additional preferred step ofthe data modulation method. The method continues from step S2 of FIG. 4and continues to step S10. Step S10 comprises interleaving the trainingsymbols among the data symbols to form several, i.e. at least three,sets of data symbols separated by respective sets of one or moretraining symbols as has previously been described. The method thencontinues to step S3 of FIG. 4, where vector precoding is applied to thenow interleaved training and data symbols.

FIG. 6 is another flow diagram illustrating an additional preferred stepof the data modulation method. The method continues from step S3 of FIG.4 and continues to step S20. Step S20 involves adding a prefix and/orpostfix, such as a cyclic prefix and/or postfix, to the precoded symbolsgenerated in step S3. The prefix/postfix is formed by appending a set ofthe last/first precoded symbol(s) from the sequence of precoded symbolsat the beginning/end of the sequence. Thus, the last/first precodedsymbol(s) is copied and placed first/last in the sequence of precodedsymbols. The method then continues to step S4 of FIG. 4.

FIG. 7 is a flow diagram illustrating a transmitting method that can bemade EGPRS/EGPRS2 compatible. The method starts in step S30 where databits enter a channel coding operation, in which training bits areincluded with the data bits. In a possible implementation similar to theprior art EGPRS/EGRPS2, the training bits can all be positioned togetherin the center of the sequence of data bits to thereby divide the databits into to equally large sub-sequences.

In a next step S31 it is determined whether the DTCP-based processingshould be backward compatible with EGPRS/EGPRS2. If backwardcompatibility is desired the method continues to step S32, where ratematching is performed to accommodate the same number of data bits thatis defined according to the EGPRS/EGPRS2 standard. The rate matching canbe performed through additional puncturing, imputation of puncturedcoded bits or repetition of coded bits depending on the currentsituation, i.e. the number of training and data bits from the channelcoding step S30. If backward compatibility is not necessary, the radioburst can accommodate a different number of data bits than theEGPRS/EGPRS2 coding schemes.

The method continues to step S33, where burst formatting is performed toadd tail and guard bits organize these together with the data andtraining bits to form a bit sequence to be transmitted during a radioburst. The bits of the bit sequence are mapped into symbols in step S34as has previously been described, e.g. in connection with step S2 ofFIG. 4. The output from the symbol mapping is the symbol sequencecomprising data, training, tail and guard symbols.

The order of steps S33 and S33 can alternatively be interchanged so thatsymbol mapping is performed before burst formation.

A next step S35 determines whether DTCP should be applied or not. Thisdecision can be based, as has previously been described, on the currentcommunication service, the mobile device capability and/or the currentsignal quality. If DTCP is to be used the method continues to step S36where vector precoding is applied to the data and training symbols toform precoded symbols as has been discussed in the foregoing, e.g. inconnection with step S3 of FIG. 4. In a preferred implementation asillustrated in FIG. 3, symbol interleaving is, however, conducted beforevector precoding. As a consequence a symbol interleaving step may beintroduced between steps S35 and S36.

The method continues from step S36 or S35 to S37 where pulse shaping isperformed. The pulse shaping can be conducted according to well knownEGPRS/EGPRS2 techniques. Alternatively, the transmit symbols from stepS36 or S35 can be modulated onto a carrier signal having a bandwidththat is a multiple of the traditional GSM channel bandwidth of 200 kHz.

The resulting analog RF signal is then upmixed and amplified in step S38and transmitted in step S39 by an antenna system during a time slothaving a length of 15/26 ms to meet the TDMA requirements of GSMsystems.

FIG. 8 is a flow diagram illustrating an embodiment of the symbolmapping step S34 of FIG. 7. The method continues from step S33 in FIG. 7and continues to step S40. In step S40 it is determined whether thesymbol mapping used in EGPRS/EGPRS2 should be employed or not. Ifaffirmative the method continues to step S41, where symbol mapping isperformed using a single symbol constellation, such as QPSK, 16 QAM, 32QAM, 64 QAM or 128 QAM. If, however, the symbol mapping must notnecessarily be according to EGPRS/EGPRS2 the method continues to stepS42. In this case multiple symbol constellation can be used within thesingle radio burst. This means that some of the symbols can be from afirst symbol constellation, whereas other bits are mapped into symbolsusing a second symbol constellation. In step S42 two or more differentsymbol constellations can be used. The method then continues from stepS41 or S42 to S35 in FIG. 7.

FIG. 9 is a flow diagram illustrating an embodiment of the pulse shapingstep S37 of FIG. 7. The method continues from step S35 or S36 in FIG. 7and continues to step S50. This step S50 is preferably only relevant ifvector precoding has been used. Thus, if vector precoding has not beenused the method continues directly to step S51. Otherwise step S50determines whether the EGPRS/EGPRS2 spectrum mask should be preservedand used. In such a case, the method continues to step S51 where theEGPRS/EGPRS2 pulse shaping filter is employed in the pulse shapingoperation. Otherwise a DTCP specific pulse shaping filter is used instep S52. This pulse shaping filer is designed to employ the bandwidthprovided by multiple adjacent 200 kHz GSM carriers. The method thencontinues from step S51 or S52 to step S38 of FIG. 7.

Simulations have been conducted comparing the prior art EGPRS2-Atechnique and an embodiment with a DTCP-based EGPRS-A compatibletechnique. In this simulation the EGPRS2-A modulation and coding schemesDAS5-DAS12 have been used with the 8-PSK, 16QAM and 32QAM modulationconstellations. The payload and channel coding from these modulation andcoding schemes, together with the following simulation assumptions havebeen used: i) 900 MHz band, ii) no transmitter impairments, iii) legacylinearized GSMK pulse shaping filter, iv) receiver impairments: phasenoise, 1.5 deg RMS, 20 kHz bandwidth, v) frequency offset: 50 Hz, vi)single branch transmitter and single branch receiver, vii) the trainingsymbols are uniformly distributed over the burst (applicable only inconnection with DTCP technique), viii) propagation mode: TU50noFH, andix) DTCP is implemented using IDFT-based vector precoding.

The results from the simulation are illustrated in the diagram of FIG.10, where the throughput in kbit/s is plotted versescarrier-to-interference ratio (CI). As is seen from the figure theembodiment employing the proposed DTCP enhancement achievessubstantially the same throughput as the prior art solution at low CIlevels, i.e. below about 10 dB. However, over about CI 10 dB theproposed DTCP enhancement provides superior throughput as compared tothe prior art techniques.

The embodiments described above are to be understood as a fewillustrative examples of the present invention. It will be understood bythose skilled in the art that various modifications, combinations andchanges may be made to the embodiments without departing from the scopeof the present invention. In particular, different part solutions in thedifferent embodiments can be combined in other configurations, wheretechnically possible. The scope of the present invention is, however,defined by the appended claims.

The invention claimed is:
 1. A modulator comprising: a symbol mapperconfigured to receive a bit sequence corresponding to a radio burst andcomprising data bits, training bits, tail bits and guard bits and to maprespective sets of at least one bit of said bit sequence into respectivesymbols to form a symbol sequence comprising data symbols, trainingsymbols, tail symbols and guard symbols; a vector precoder configured toapply a vector precoding transformation to said data symbols and saidtraining symbols without applying the vector precoding transformation tosaid tail symbols and said guard symbols to form corresponding precodedsymbols of said data symbols and said training symbols; a processorconfigured to add one of a prefix and a postfix to the precoded symbols;and a symbol processor configured to form a sequence of transmit symbolscomprising said tail symbols and said guard symbols from said symbolmapper and said precoded symbols from said vector precoder.
 2. Themodulator according to claim 1, wherein said symbol mapper is furtherconfigured to map respective sets of multiple bits of said bit sequenceinto respective symbols to form said symbol sequence comprising saiddata symbols, said training symbols, said tail symbols and said guardsymbols.
 3. The modulator according to claim 1, further comprising asymbol interleaver configured to interleave said training symbols amongsaid data symbols to form at least Q≧3 sets of said data symbolsseparated by respective sets of at least one of said training symbols.4. The modulator according to claim 1, wherein said vector precoder isfurther configured to apply one of a discrete Fourier transformation, aninverse discrete Fourier transformation, a discrete cosinetransformation, an inverse discrete cosine transformation, a discretewavelet transformation and an inverse discrete wavelet transformation tosaid data symbols and said training symbols to form said correspondingprecoded symbols of said data symbols and said training symbols.
 5. Themodulator according to claim 4, wherein said symbol mapper is furtherconfigured to generate a symbol vector z=[z₁, . . . , z_(N)]^(T)comprising said data symbols and said training symbols, where Nrepresents a total number of said data symbols and said trainingsymbols; and said vector precoder is further configured to apply an N×Ninverse discrete Fourier transform matrix w to said symbol vector toform said precoded symbols comprising said data symbols and saidtraining symbols.
 6. The modulator according to claim 4, wherein saidsymbol mapper is further configured to generate a symbol vector z=[z₁, .. . , z_(N)]^(T) comprising said data symbols and said training symbols,where N represents a total number of said data symbols and said trainingsymbols; and said vector precoder is further configured to apply an N×Ninverse discrete cosine transform matrix w to said symbol vector to formsaid precoded symbols comprising said data symbols and said trainingsymbols.
 7. The modulator according to claim 1, wherein said precodedsymbols from said vector precoder form a vector Z=[Z₁, . . . , Z_(N)]and wherein said processor configured to add one of said prefix and saidpostfix to said precoded symbols comprises a cyclic prefix processorconfigured to add a cyclic prefix to said precoded symbols from saidvector precoder by appending a last L precoded symbols from said vectorprecoder in said vector Z at a beginning of said vector z to form a newvector Z_(P)=[Z₁ ^(P), . . . , Z_(N+L) ^(P)]^(T)=[Z_(N−L), . . . Z_(N),Z₁, . . . , Z_(N)]^(T).
 8. The modulator according to claim 1, whereinsaid symbol processor is further configured to form said sequence oftransmit symbols comprising said precoded symbols from said vectorprecoder flanked by said tail symbols and flanked by said guard symbols.9. The modulator according to claim 1, wherein said symbol mapper isfurther configured to: map, for each said data symbol x_(i), i=1, . . ., D, a set of at least one said data bit into said data symbol drawnfrom a symbol constellation; map, for each said training symbol s_(i),i=1, . . . , N_(tr), a set of at least one said training bit into saidtraining symbol drawn from a symbol constellation; map, for each saidtail symbol t_(i), i=1, . . . , ν, a set of at least one said tail bitinto said tail symbol drawn from a symbol constellation; and map, foreach said guard symbol g_(i), i=1, . . . , η, a set of at least one saidguard bit into said guard symbol drawn from a symbol constellation. 10.The modulator according to claim 1, wherein said symbol mapper isfurther configured to map respective sets of at least one bit of saidbit sequence into respective symbols to form a symbol sequence ofquadrature amplitude modulation (QAM), and/or phase-shift keying (PSK)data symbols, training symbols, tails symbols and guard symbols.
 11. Themodulator according to claim 1, wherein said symbol mapper is furtherconfigured to select, for each said respective set of at least one bit,a symbol constellation selected from a quadrature amplitude modulation(QAM) symbol constellation and phase-shift keying (PSK) symbolconstellation based on a position of said respective set within said bitsequence, wherein a first symbol constellation selected for a first saidset of at least one bit is different from a second symbol constellationselected for a second said set of at least one bit.
 12. The modulatoraccording to claim 1, wherein said symbol mapper is further configuredto map said respective sets of multiple bits of said bit sequence intorespective symbols to form a symbol sequence comprising 156 symbols intotal and 6 tails symbols, 8 guard symbols and 142 data and trainingsymbols.
 13. The modulator according to claim 1, further comprising apulse shaper configured to modulate said sequence of transmit symbolsonto a carrier signal having a bandwidth of M×200 kHz, where M is apositive integer.
 14. A base station implementable in a radio-basedcommunication network comprising: a modulator, comprising: a symbolmapper configured to receive a bit sequence corresponding to a radioburst and comprising data bits, training bits, tail bits and guard bitsand to map respective sets of at least one bit of said bit sequence intorespective symbols to form a symbol sequence comprising data symbols,training symbols, tail symbols and guard symbols; a vector precoderconfigured to apply a vector precoding transformation to said datasymbols and said training symbols without applying the vector precodingtransformation to said tail symbols and said guard symbols to formcorresponding precoded symbols of said data symbols and said trainingsymbols; a processor configured to add one of a prefix and a postfix tothe precoded symbols; and a symbol processor configured to form asequence of transmit symbols comprising said tail symbols and said guardsymbols from said symbol mapper and said precoded symbols from saidvector precoder; and a transmit antenna configured to transmit a radioburst carrying said sequence of transmit symbols during a time slothaving a length of 15/26 ms.
 15. A data modulation method comprising:inputting a bit sequence corresponding to a radio burst and comprisingdata bits, training bits, tail bits and guard bits; mapping respectivesets of at least one bit of said bit sequence into respective symbols toform a symbol sequence comprising data symbols, training symbols, tailsymbols and guard symbols; applying a vector precoding transformation tosaid data symbols and said training symbols without applying the vectorprecoding transformation to said tail symbols and said guard symbols toform corresponding precoded symbols of said data symbols and saidtraining symbols; adding one of a prefix and a postfix to the precodedsymbols; and forming a sequence of transmit symbols comprising said tailsymbols and said guard symbols and said precoded symbols.
 16. The methodaccording to claim 15, wherein mapping respective sets comprises mappingrespective sets of multiple bits of said bit sequence into respectivesymbols to form said symbol sequence comprising said data symbols, saidtraining symbols, said tail symbols and said guard symbols.
 17. Themethod according to claim 15, further comprising interleaving saidtraining symbols among said data symbols to form at least Q≧3 sets ofsaid data symbols separated by respective sets of at least one of saidtraining symbols.
 18. The method according to claim 15, furthercomprising generating a symbol vector z=[z₁, . . . , z_(N)]^(T)comprising said data symbols and said training symbols, where Nrepresents a total number of said data symbols and said trainingsymbols, wherein applying said vector precoding transformation comprisesapplying an N×N inverse Fourier transform matrix w to said symbol vectorto form said precoded symbols comprising said data symbols and saidtraining symbols.
 19. The method according to claim 15, furthercomprising generating a symbol vector z=[z₁, . . . , z_(N)]^(T)comprising said data symbols and said training symbols, where Nrepresents a total number of said data symbols and said trainingsymbols, wherein applying said vector precoding transformation comprisesapplying an N×N inverse discrete cosine transform matrix w to saidsymbol vector to form said precoded symbols comprising said data symbolsand said training symbols.
 20. The method according to claim 15, furthercomprising: generating a symbol vector z=[z₁, . . . , z_(N)]^(T)comprising said data symbols and said training symbols, where Nrepresents a total number of said data symbols and said trainingsymbols; and wherein adding one of said prefix and said postfix to theprecoded symbols comprises adding a cyclic prefix to said precodedsymbols by appending a last L precoded symbols in said vector z at abeginning of said vector z to form a new vector Z^(P)=[Z₁ ^(P), . . . ,Z_(N+L) ^(P)]^(T)=[Z_(N−L), . . . Z_(N), Z₁, . . . , Z_(N)]^(T).
 21. Themethod according to claim 15, wherein mapping respective sets comprises:selecting, for each said respective set of at least one bit, a symbolconstellation from a quadrature amplitude modulation(QAM) symbolconstellation and phase-shift keying (PSK) symbol constellation based ona position of said respective set within said bit sequence; and mappingsaid set of at least one bit to a symbol drawn from said selected symbolconstellation, wherein a symbol constellation selected for a first saidset of at least one bit is different from a symbol constellationselected for a second said set of at least one bit.
 22. The methodaccording to claim 15, further comprising: modulating said sequence oftransmit symbols onto a carrier signal having a bandwidth of M×200 kHz,where M is an positive integer; and transmitting a radio burst carryingsaid sequence of transmit symbols during a time slot having a length of15/26 ms.